X-ray Interactions Photoelectric and Compton Scattering for Radiologic Technologists [Rad Physics]

so when i was growing up if someone would have told me we're going to be talking about photoelectric and compton i would have thought we were talking about serena williams venus williams because they were photoelectric on the court and they came straight out of compton hey guys i'm brian from howardeologyworks.com we have bite size information that's of interest for those in the radiology field if that sounds good to you click subscribe below and then click on the little bell icon so you can get notified when we release new content but we're not talking about the william sisters today or their dad richard we're actually going to be talking about photoelectric and compton interactions so we're going to go through that we're going to go through pictorially and if we remember here you have the nucleus and that nucleus has an atomic number z or how many protons are in that nucleus and then here we have an electron cloud and in general the electron cloud is much larger than the nucleus so then the nucleus where the protons and neutrons reside takes up a very small fraction of the matter in general so you have your x-ray photons coming in and in this case those x-ray photons are going to be interacting with an inner shell electron so an electron which is close to the nucleus then if you remember from chemistry we we have these different shells where the electrons reside and the shells that are closer have lower energy and the shells that are a little bit further away they have a little bit higher energy or rather they have higher energy they're not necessarily physically further away but those shells have higher energy and when an electron is removed from an inner shell there's a vacancy and then an outer shell electron will want to reach a more stable or lower energy configuration so it'll move from an outer shell to an inner shell and in order to conserve energy it's going to emit in this case a photon so we'll go through and we'll watch that in this little cartoon so our x-ray comes in it knocks out the electron so we have an electron knocked out and then this electron moves down and then a characteristic secondary x-ray is emitted but what's important to remember is that right here we have an x-ray and an electron but those are both going to deposit their energy fairly locally so we can think about it to first order as the x-ray comes in it interacts with the matter and then it deposits its energy locally so this is nice from the perspective of if we want to generate contrast in an image it's nice that we can have some energy deposited locally and then we can basically make a map of that we'll talk about that in additional lectures but one thing to keep in mind here is that this was an inner shell electron that was having this interaction and that's very much um driven by what's in the nucleus namely what's the atomic number how many protons are in the nucleus so the likelihood of this interaction goes like z cubed or how many protons are in the nucleus cube so things that have more protons are going to have higher contribution of photoelectric effects when they're interacting so we'll keep that in mind then the other thing is compton scattering so content scattering is the second um dominant effect in x-ray imaging so in this case again we have the nucleus and then we have an electron cloud in this case it's the photon coming in and the photons interacting with an outer shell electron and because it's so far from the nucleus the likelihood of the interaction is independent of z so it's not really dependent on what's in the nucleus photon comes in it knocks the electron out and then the photon goes out in the opposite direction in order to conserve momentum so what's important to remember here is that unlike in the photoelectric effect the energy is not all deposited locally so this scattered photon may still have a significant fraction of the energy of the incoming photon and that can still travel through the patient and potentially could have a secondary scatter effect or potentially could get measured on the detector so we've covered the two major interactions photoelectric and compton if we look at the relative contribution of those two we can see a plot like this where this is the relative contribution so these right here are saying at low energies it's all the interactions are really dominated by photoelectric photoelectric is in green here and compton is in red and then at high energies everything is dominated by compton and that's true for these different types of material the body can well be approximated as a bag of water for the soft tissue sake and then the bone in the body here shows a similar behavior but the point at which they cross over is a little bit higher so photoelectric is dominant for more of the time in bone than in water so up to 26 kev photoelectric is dominant in water and it's up to 45 kev that photoelectric is dominant in bone why is that we just talked about the fact that the photoelectric effect is strongly dependent on the z and bone has a much higher z for instance the calcium in the bone has a much higher z many more protons than in water so that's the fundamental uh relationship that we want to try and remember is at low energy photoelectric is dominant and high energy compton's dominant so just again to summarize really take these points home photoelectric dominates interactions at low energies compton will dominate interactions at higher energies and then the likelihood of these interactions for photoelectric it's dependent on z cubed and for content it's dependent on z and what what does that tell us it tells us that x-rays and x-ray imaging we're going to have really good contrast for things that are higher z like bone so bone imaging on standard x-rays comes out really nice with really fine contrast so thanks for sticking around guys and we'll see you in the next one it's a summary on x-ray interactions with matter you

Channel: How Radiology Works

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Keywords: x-ray interactions photoelectric and compton scattering, Photoelectric X-ray, X-ray Photoelectric, Compton Scattering, Photoelectric vs Compton Scattering, radiologic technologist, xray tech, radiology student, rad tech, x ray tech, x ray technologist, rad tech school, rad tech student, rad tech program, radiologic technologist review, rad tech boot camp, x ray tech tips, radiologic technologist vlog, radiology for medical students, Radiologic technologist student

Id: OaQHP4jD_hE

Length: 6min 7sec (367 seconds)

Published: Tue Jan 05 2021